Electronic users continue to demand ever smaller, more portable devices, such as hand-held mobile phones and personal computers as well as smart cards and a host of other electronic devices. For portability, these electronic devices may carry their own power supply or be used with an adapter that, for example, allows such a device to be connected to a wall socket. The latter approach usually makes use of such a device more cumbersome, while the former approach may constrain how small the device may be if, for example, a relatively large portable battery must be used as the internal power source.
Limiting the size of such a portable electronic device's internal power source typically calls for reducing the device's power consumption—especially, for example, during times when the device is idle. A device user, nonetheless, typically desires that the device be capable of being powered up quickly and readily usable even after the device has been idle for an extended period of time. Thus, a power control circuit may be interposed between a portable power supply and the operational circuitry of the device. The power control circuit may control the supply of power by reducing the supply of power to the operational circuit when the device is idle so that overall power consumption is thereby reduced.
One approach is to include as part of the power control circuit a wake-up sensor that senses an external stimulus that generally corresponds to a user's intent to use the device, the power control circuit responding to the sensor by powering up the device so that the operational circuitry goes into an active mode. U.S. Pat. No. 6,469,639 to Tanenhaus et al., for example, discloses that the wake up sensor may be a micro-electro-mechanical (MEM) accelerometer, which senses the acceleration of the device when it is picked up or moved by a user. More particularly, the accelerometer is a low-powered, integrated circuit or “MEMS chip” that provides a sensing signal to a buffering circuit, which, in turn, is connected to a threshold detecting circuit for detecting whether and when the buffered sensing signal reaches or exceeds a predetermined threshold value.
The MEMS accelerometer may be used for other devices as well. For example, a so-called “smart gun” developed by the New Jersey Institute of Technology includes a MEMS accelerometer that determines when the gun has been picked up and accordingly wakes up other circuitry, which goes into a sleep mode when the gun is at rest. The other circuitry includes pressure sensors that, when powered up, determine whether the size of the hand holding the gun matches stored biometric data.
A conventional wake-up sensor, such as the MEMS accelerometer, frequently operates by detecting changes in voltage and/or current levels. Accordingly, such a conventional wake-up sensor usually needs to be powered even when the device with which it is used is in idle mode. This, however, also consumes power and reduces battery life for battery-powered electronics.